An important element in high-speed fiber-optic transmission systems is the ability to optically encode data bits for transport in optical fiber media. One way this is achieved is through the modulation of the output of a continuous-wave laser source by an electro-optical modulator, whose output is coupled to an optical fiber for transmission. Many applications require high quality optical modulation performance, which imposes amplitude and signal quality requirements on the drive electronics, often referred to as a modulator driver, required to interface with the electro-optical modulator. Additionally, as optical network data rates increase, many applications require the electrical modulator driver to maintain the appropriate signal requirements for achieving high quality optical modulation performance at higher data rates.
FIG. 1 illustrates the top view of typical electro-optical modulator integrated circuit known in the art which is capable of providing modulation of an optical signal, based on a Mach-Zehnder interferometer technique with single-ended electrical drive input. A continuous-wave optical signal is input to an optical waveguide 12 where it is split into two paths. An electrical data signal from a single-ended modulator driver is input to the RF IN port where it travels along an electrical transmission line 14 between the optical waveguide paths, and creates an electric field between the transmission line and two ground electrodes 21, 22. Due to the geometry of the layout, the electric field distribution will have opposite polarity in each of the optical waveguides, producing a change in the phase in each of the optical waveguides that has opposite direction. With a sufficiently large electrical signal amplitude, typically 4 to 8 volts peak-to-peak, the phase shifts induced in the optical waveguide paths, when combined, will cause the optical output signal to be modulated.
A large amplitude drive signal is difficult to achieve at high-speed data rates due to the parasitics of the transistor device sizes required to generate the large electrical output voltage swings. One method known in the art for overcoming the bandwidth limitations imposed by the device parasitics is the use of a single-ended distributed amplifier. A typical single-ended distributed amplifier topology is illustrated in FIG. 2. In this topology, an input signal travels along an artificial transmission line formed by inductive elements 31 and the input capacitance of transistor amplifier stages 40. The traveling wave input signal is amplified by the transistor amplifier stages 40 which in turn output signals onto an output artificial transmission line formed by inductive elements 32 and the output capacitance of the transistor amplifier stages 40. The output signals will have a forward and reverse traveling wave component, where the reverse traveling wave component is terminated by the termination 35, and the forward traveling wave signal will be output from the distributed amplifier. A bias-T structure 42 is typically disposed at the output port and used to provide access to a positive supply voltage VCC for biasing of the transistor amplifier stages 40.
While the topology in FIG. 2 can provide wideband amplification and large output amplitude capability, it has several limitations when used for modulator driver applications. One limitation is that adjustment of the output signal amplitude and duty cycle can be difficult, typically requiring external circuitry for simultaneous adjustment of the supply voltage VCC and gate bias voltage VG. Another limitation is that the gain of this distributed amplifier topology is typically lower than required for single-ended modulator driver applications. To achieve higher gain, multiple single-ended distributed amplifiers 60, 61 are typically used in series, such as illustrated in FIG. 3, which adds complexity, size and cost. A further limitation is that stabilization of the output signal amplitude over a range of input signal amplitudes can be difficult, typically requiring amplifier saturation which can cause poorly controlled output signal rise and fall times. Finally, typical inter-stage bias-T components for the broadband transmission of data are inconsistent with monolithic IC fabrication techniques, which can result in the use of bulky multi-chip modules to achieve the modulator driver function.
Accordingly, it would be desirable to have a single-ended output modulator driver architecture capable of high output amplitude, high gain, and high-speed data transmission and compatible with compact, monolithic process fabrication techniques with a minimum of external components required for operation. In addition, it would be desirable to have a single-ended output modulator driver architecture with input limiting function capable of providing a stabilized output signal amplitude over a range of input signal amplitudes. Furthermore, it would be desirable to have a single-ended output modulator driver architecture with simple control circuitry for output amplitude and output duty-cycle adjustment.